Low-density molding compound

ABSTRACT

A molding composition formulation includes a thermoset cross-linkable polymeric resin. Glass microspheroids are present such that upon cure of the resin the resulting article has a linear shrinkage of less than ±0.06% and a density of less than 1.65. An article formed from such a composition is further strengthened by the addition of a surface activating agent bonded to the surface of the glass microspheroids. Conventional particulate fillers when added to an inventive formulation provide enhanced performance when the filler particle has a size sufficiently small to insert within adjacent microspheroid interstitial voids. An unsaturated polyester resin so formed is particularly well suited for the formation of sheet molding compound formulations.

RELATED APPLICATION

This application claims priority of U.S. Provisional Patent ApplicationSer. No. 60/680,893 filed May 13, 2005, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention in general relates to low density sheet moldingcompounds and in particular, to a low-density sheet molding compoundsandable to a high sheen surface.

BACKGROUND OF THE INVENTION

In order to reduce the density of a sheet molding compound (SMC) or bulkmolding compound (BMC), high-density inorganic fillers, such as calciumcarbonate, are removed from the formulation and replaced withthermoplastic in an effort to maintain surface quality characteristics.Another approach common to the field is to remove a portion of theinorganic filler and replace some of the inorganic filler with theindustry standard SMC-1 hollow glass microspheres. Spherical particulateunder processing conditions have a tendency to become displaced from themolding compound matrix. Additionally, under the stresses associatedwith sanding a molded article, such hollow glass spheres have a tendencyto rupture, leaving a cavity in the molded article that is amenable todisintegration. The ruptured hollow glass sphere is manifest in apainted article as a paint pop that effectively destroys the surfacefinish. Due to these problems, hollow glass spheres have not beenacceptable in SMC or BMC molded articles requiring a class “A” surfacefinish as defined by Powder Prime Patent. As class “A” surfaces areroutinely required in vehicle components, the ability to form a lowdensity class “A” SMC article would lower the vehicle weight and therebyincrease fuel efficiency.

Thus, there exists a need for a low-density class “A” SMC or BMCcontaining hollow glass microspheres that retains class “A” surfacefinish characteristics.

SUMMARY OF THE INVENTION

A molding composition formulation according to the present inventionincludes a thermoset cross-linkable polymeric resin. Glassmicrospheroids are present such that upon cure of the resin theresulting article has a linear shrinkage of less than ±0.06% and adensity of less than 1.65. Solid glass microspheroids having a meandiameter of from 16 to 35 microns are particularly effective. An articleformed from such a composition is further strengthened by the additionof a surface activating agent bonded to the surface of the glassmicrospheroids. Additionally, conventional particulate fillers whenadded to an inventive formulation provide enhanced performance when thefiller particle has a size sufficiently small to insert within adjacentmicrospheroid interstitial voids.

A process for producing an article from a molding compositionformulation and retaining surface quality includes adding a surfaceactivating agent coated glass microspheroid having a mean particlediameter of from 16 to 35 microns to a molding composition formulationcontaining a thermoset cross-linkable polymeric resin. Upon allowingsufficient time for the resin to cross link, a molded article isproduced with retention of high surface quality. An unsaturatedpolyester resin is particularly well suited for the formation of sheetmolding compound formulations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph of a conventional 85 micronhollow glass microsphere shattered in the course of sanding the plateausurface. The distance between adjacent dots corresponds to 5 microns;and

FIG. 2 is a scanning electron micrograph of the same base SMCformulation containing an 18 micron hollow glass microsphere subjectedto the same sanding regimen as that depicted in FIG. 1. The distancebetween adjacent dots corresponds to 5 microns.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has utility as a molding compound. According tothe present invention, a microspheroid having a mean diameter of between10 and 120 microns is introduced into a base SMC or BMC formulation toafford a class “A” surface finish.

As used herein a microspheroid is defined to include a hollowmicrosphere or a solid bead having an aspect ratio of between two normaldiameters of between 1 and 1.6. Typically, a spheroid particle is formedof glass or a thermoplastic material.

A class “A” surface as used herein is defined to be a cured SMC or BMCmaterial having a linear shrinkage of ±0.06%. Such a material containingunsaturated polyester resin, thermoplastic additives, organic peroxides,inhibitor, filler, mold release and pigment is sold by ThyssenKrupp Buddunder the trade name TCA®.

Preferably, the microspheroids have a mean diameter of between 12 and 45microns. Most preferably, the microspheroids have an outer dimension ofbetween 16 and 35 microns. Typically, microspheroids are loaded into abase SMC or BMC class “A” formulation from 2 to 12 total weight percentof the resulting formulation. Preferably, the microspheroids are presentfrom 4 to 6 total weight percent of the resulting SMC or BMCformulation. The specific amount of microspheroids added into a givenmolding composition formulation is dependent on factors includingdesired article density, microspheroid size dispersion and mean particledimension, required article strength, required article shrinkage, andrequired article surface smoothness.

In a particularly preferred embodiment of the present invention, themicrospheroids are pretreated with a surface coating adherent to themicrospheroid surface.

A microspheroid surface is readily derivatized to bond to a surroundingresin matrix during cure. The resulting article exhibits improvedphysical properties. One type of surface derivative for a microspheroidis a heteroatom functionally terminated thermoplastic coating. Theheteroatom containing terminus illustratively includes a tertiaryamine-, hydroxyl-, imine- or cyano-moiety. It is appreciated that suchmoieties under appropriate cure conditions known to the art are able toreact with matrix resin components during cure to further strengthen acured article. Tertiary amine terminated thermoplastic are readilyprepared. D. H. Richards, D. M. Service, and M. J. Stewart, Br. Polym.J. 16, 117 (1984). A representative tertiary amine terminatedthermoplastic is commercially available under the trade name ATBN 1300 X21 from Noveon.

A surface activating agent that bonds to a glass microspheroid is analkoxysilane where the silane is reactive with the silica surface of themicrospheroid. Representative alkoxysilane surface activating agents forthe microspheroid illustratively include: 3-aminopropyltrimethoxysilane,3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane,3-glycidoxypropyltriethoxysilane,(3-glycidoxypropyl)bis(trimethylsiloxy)methylsilane,(3-glycidoxypropyl)methyldiethoxysilane,(3-glycidoxypropyl)dimethylethoxysilane,(3-glycidoxypropyl)methyldimethoxysilane,methacryloxymethyltriethoxysilane, methacryloxymethyltrimethoxysilane,methacryloxypropyldimethylethoxysilane,methacryloxypropyldimethylmethoxysilane,ethacryloxypropylmethyldimethoxysilane,methacryloxypropyltriethoxysilane, methoxymethyltrimethylsilane,3-methoxypropyltrimethoxysilane,3-methacryloxypropyldimethylchlorosilane,methacryloxypropylmethyldichlorosilane,methacryloxypropyltrichlorosilane,3-isocyanatopropyldimethylchlorosilane,3-isocyanatopropyltriethoxysilane,bis(3-triethoxysilylpropyl)tetrasulfide, and combinations thereof. Morepreferably, the silane surface activating agent includes an ethenicallyunsaturated moiety that is reactive under free radical cross-linkingconditions so as to covalently bond with the hollow glass microsphere tothe surrounding SMC or BMC class “A” matrix.

Alternatively, it is appreciated that microspheroid surface activatingagent is readily mixed into the pre-cured SMC or BMC formulation andhollow glass microspheres added thereto to induce microsphere activationprior to initiation of matrix cure. Typically, the surface activatingagent is present in concentrations of about 0.05 to 0.5 grams of surfaceactivating agent per gram of microspheres.

A principal component of an SMC or BMC formulation is a cross-linkablepolymer resin such as an unsaturated polyester resin or vinyl esterresin. The prepolymer polymeric resin has a molecular weight on averageof typically between 400 and 100,000 Daltons. The polyester prepolymerresins typically represent condensation products derived from thecondensation of unsaturated dibasic acids and/or anhydrides withpolyols. It is appreciated that the saturated di- or poly-acids are alsopart of the condensation process to form polyester prepolymers with alesser equivalency of reactive ethylenic unsaturation sites. Typical SMCand BMC formulations are described in U.S. Pat. Nos. 4,260,358;4,643,126; 5,100,935; 5,268,400; 5,854,317 and 6,780,923. A particularlypreferred class “A” SMC formulation is detailed in pending U.S. patentapplication Ser. No. 11/037,908 filed Jan. 18, 2005.

Vinyl ester resins are also typically employed in SMC or BMCformulations as a polymeric resin. Vinyl ester prepolymer resins aretypically the reaction product of an epoxy resin with a carboxylic acidhaving a single ethylenic unsaturation. Specific vinyl ester resinscommonly used are the reaction product of epoxy functionalized bisphenolA with an acrylic acid. As a result of the difference in prepolymersynthesis, the vinyl ester resin prepolymers are typically associatedwith terminal ethylenic unsaturations while polyester resin prepolymerspredominantly have ethylenic unsaturations internal to the prepolymerbackbone.

The polymeric resin prepolymer is suspended, and preferably dissolved,in an ethylenically unsaturated monomer that copolymerizes with theresin during the thermoset process. It is appreciated that more than onetype of monomer can be used in a molding composition. The monomerprovides benefits including lower prepolymer viscosity and thermosettingwithout formation of a volatile byproduct. Monomer is typically presentup to 18.5 total weight percent of a molded article.

A typical molding composition includes a free radical initiator toinitiate cross-linking between the polymeric prepolymer resin withitself or with ethylenically unsaturated monomer, if present. A freeradical initiator is typically chosen to preclude significantcross-linking at lower temperature so as to control the thermosetconditions. Conventional free radical polymerization initiators containeither a peroxide or azo group. Peroxides operative hereinillustratively include benzoyl peroxide, cyclohexanone peroxide,ditertiary butyl peroxide, dicumyl peroxide, tertiary butyl perbenzoateand 1,1-bis(t-butyl peroxy)3,3,5-trimethylcyclohexane. Azo speciesoperative herein illustratively include azobisisobutyronitrile andt-butylazoisobutyronitrile. While the quantity of free radicalpolymerization initiator present varies with factors such as desiredthermoset temperature and decomposition thermodynamics, an initiator istypically present from 0.1 to 3 total weight percent. In order to lessencross-linking at temperatures below the desired thermoset temperature, apolymerization inhibitor is often included in base molding formulations.Hydroquinone and t-butyl catechol are conventional inhibitors. Aninhibitor is typically present between 0 and 1 total weight percent.

The inventive molding composition preferably includes a particulatefiller. Particulate fillers operative in such molding compositionsillustratively include calcium carbonate, calcium silicate, alumina,ATH, silica, talcs, dolomite, vermiculite, diatomaceous earth, kaolinclay, graphite, metal and combinations thereof. Factors relevant in thechoice of a particulate filler illustratively include filler cost,resultant viscosity of flow properties, resultant shrinkage, surfacefinish weight, flammability, electrical conductivity, and chemicalresistance of the thermoset formulation. Particulate filler typicallyaccounts from 0 to 80 weight percent of the molding composition totalweight. Typical filler sizes are from 0.1 to 50 microns.

In a preferred embodiment, the filler particle size is matched to thatof the microspheroid such that interstitial filling of voids associatedwith one of the filler or microspheroids is filled by the other. Morepreferably, the microspheroids are larger than the filler particle sizeto an extent that a filler particle can reside in an interstice betweenclosely packed microspheroids. By way of example, four coplanar 18micron solid glass microspheroids packed in an article formed from aninventive molding composition matrix to create a 7.4 micron diameterinterstice wherein four conventional 85 micron glass spheres are packedtogether to yield a 35 micron diameter interstice. A calcium carbonatefiller having a mean 5 micron diameter is well suited to fillinterstitial spaces formed between microspheroids of the presentinvention. One of skill in the art can readily calculate interstitialdimensions by geometric techniques associated with crystallography.Assuming a microspheroid average radius of r for a group of contiguousmicrospheroids forming a four spheroid intersective interstice, the sizeof a filler particle capable of filling the interstice is less than orequal to a diameter D given by:D≦2√{square root over (2 r ²)}−2 r

While not intending to be bound by a particular theory, it is surmisedthat interstitial dispersion of small particles within a grouping oflarger particles inhibits formation of an inhomogenous region rich infiller. Inhomogenous filler regions with comparatively weak interactionswith a surrounding cured matrix are believed to promote crackpropagation and thereby weaken the resulting article.

It is appreciated that curable compositions other than SMCs and BMCsbenefit from the inclusion of microspheroids sized to incorporateconventional filler particles into interstices between adjacentmicrospheroids. A microspheroid-filler size ratio to provideinterstitial packing is employed in curable compositions alsoillustratively including epoxies, polyurethanes and polyureas.

A fiber filler is typically added to provide strength relative to aparticulate filler. Fiber fillers operative herein illustrativelyinclude glass, carbon, polyimides, polyesters, polyamides, and naturalfibers such as cotton, silk, and hemp. Preferably, the fiber filler isglass fiber in the form of chopped glass strands. More preferably,chopped glass strands are provided in lengths ranging from 5 to 50millimeters. Fiber fillers are typically present from 0 to 80 totalweight percent.

A mold release agent is typically provided to promote mold release. Moldreleases include fatty acid salts illustratively including oleates,palmitates, sterates of metal ions such as sodium, zinc, calcium,magnesium, and lithium. A mold release is typically present from 0 to 5total weight percent.

A low profile additive is optionally provided to improve surfaceproperties and dimensional stability of a resulting molded product. Lowprofile additives illustratively include thermoplastics and elastomerssuch as polyethylene, polystyrene, polyvinyl toluene, polyacrylates,polyethylene ether polymers, polyphenylene oxide polymers, andcombinations thereof. Copolymers operative as low profile additivesinclude the above-recited polymers in addition to copolymers of the sameincluding butadiene, acrylonitrile, and vinyl chloride. Low profileadditives are typically present from 0 to 50 total weight percent andmore often from 5 to 40 total weight percent.

A nanocomposite clay is lipophilic and has domains of a size that do notcontribute to the molded article surface roughness. An alkyl quaternaryammonium bentonite nanocomposite clay operative herein is commerciallyavailable (Southern Clay Products, CLOISITE® 10A). Clay domains are onthe order of 50 to 150 nanometers and have a platelet thickness of oneto 10 nanometer and are optionally used alone, or in combination with apolymeric moisture reducing additive to enhance the processability of aninventive formulation while decreasing the moisture absorption rate.Such nanocomposite clay is typically present up to 10 total weightpercent.

It is appreciated that the present invention optionally alsoincorporates additional additives illustratively including flameretardants, plasticizers, colorants, and other processing additivesconventional to the art.

Molding compositions of the present invention are well suited for therapid production of molded composite articles that have a class “A”finish and a lower density than that achieved with conventional SMC-1hollow glass microspheres or the replacement of inorganic fillers withthermoplastic. The present invention is particularly well suited for theproduction of a variety of products illustratively including bumperbeams, vehicle door panel components, automotive floor components,spoilers and hoods; and various industrial and consumer producthousings.

The reduction of article density with the maintenance of surface finishproperties of a class “A” article is achieved by allowing for theaddition of lower density fiber and particulate fillers. Preferably, asheet molding composition according to the present invention has adensity of less than 1.65. Unlike the use of conventional 85 micronhollow glass microspheres that collapse under stress, the inventivemicrospheroids are preferably surface bonded to the resin matrix. Thiscoupled with interstitial fill with matched size fillers reduces thestructural demands on the filler and allows less rigorous demands on thefiller properties.

The present invention is further illustrated with respect to thefollowing non-limiting examples:

EXAMPLE 1

18 micron outer diameter glass microspheroids are mixed into aconventional class “A” sheet molding compound formulation containingunsaturated polyester resin, thermoplastic additives, organic peroxides,inhibitor, calcium carbonate filler, mold release and pigment, as soldby ThyssenKrupp Budd under the trade name TCA®. The resultingformulation is cured into a plaque having dimensions of 20 cm×20 cm×0.5cm.

COMPARATIVE EXAMPLE

Example 1 was repeated with the replacement of the 18 micron glassmicrospheroids with industry standard SMC-1 hollow glass microsphereshaving an average particle diameter of 85 microns.

EXAMPLE 2

The plaques of Example 1 and the comparative example were sanded with ahand orbital sander having a clean piece of 300 grit sandpaper over thesurface for three minutes. A central disk of plaque having a diameter of1 cm was cut from the plaque and shadowed with platinum-gold alloy. Theresulting shadowed sample was subjected to scanning electron microscopy.FIG. 1 depicts the comparative 85 micron hollow glass microsphere thatis noted to have shattered, leaving an optically perceptible defect. Incontrast, the sanded 18 micron glass microspheroid containing plaque,per Example 1, is intact and therefore retains class “A” status whilereducing the overall density of the plaque compared to a comparableplaque formed from TCA® lacking hollow glass microspheroids.

EXAMPLE 3

10 micron solid glass microspheroids are dispersed in water (33 percentweight by weight). A methanol solution of3-isocyanatopropyltriethoxysilane (10% silane by weight) is combinedwith the glass microsphere dispersion such that there are 20 silanemolecules per each glass microspheroid in the dispersion. The solutionis stirred in a polypropylene beaker overnight. The surface activatedglass spheres are decanted and dried before inclusion in theconventional sheet molding compound of Example 1 in place of the 18micron glass microspheroids. Following sanding of the resulting plaque,cured SMC matrix was noted to wet the glass microspheroid surface, withmicrographs being comparable to those depicted in FIG. 2.

Patent documents and publications mentioned in the specification areindicative of the levels of those skilled in the art to which theinvention pertains. These documents and publications are incorporatedherein by reference to the same extent as if each individual document orpublication was specifically and individually incorporated herein byreference.

1. A molding composition formulation comprising: a thermosetcross-linkable polymeric resin; and glass microspheroids with a meandiameter from 16 to 35 microns that upon cure of said resin yield anarticle having a linear shrinkage of less than ±0.06%, a density of lessthan 1.65, and the smoothness of a class-A surface finish.
 2. Theformulation of claim 1, wherein said glass microspheroids are presentfrom 4 to 6 total weight percent of the formulation.
 3. The formulationof claim 1 wherein said glass microspheroids are solid beads.
 4. Theformulation of claim 1, further comprising a surface activating agentbonded to the surface of said glass microspheroids.
 5. The formulationof claim 4, wherein said surface activating agent is a silane.
 6. Theformulation of claim 4 wherein said surface activating agent furthercomprises a cross-linkable moiety reactive during the cure of saidresin.
 7. The formulation of claim 4 wherein said surface activatingagent is a heteroatorn functionally terminated thermoplastic coating. 8.The formulation of claim 7 wherein said coating has a heteroatomcontaining terminus selected from the group consisting of: a tertiaryamine-, hydroxyl-, imine- or cyano-molety.
 9. The formulation of claim 1further comprising filler particles such that a filler particle has aparticle size that is less than or equal to a diameter D and resideswithin an interstice formed by proximal glass microspheroids accordingtoD≦2√{square root over (2 r ²)}−2 r where r is the average radius of fourcontiguous microspheroids.
 10. The formulation of claim 1 wherein saidglass microspheroids have an aspect ratio of normal diameters of between1 and 1.6.
 11. The formulation of claim 1 wherein said resin is apolyester.
 12. The formulation of claim 1 wherein said resin is selectedfrom the group consisting of: an epoxy and a polyurethane.
 13. A moldingcomposition formulation comprising: an unsaturated polyestercross-linkable resin; glass microspheroids having a mean diameter offrom 16 to 35 microns; filler particles having a size that is less thanor equal to a diameter D and resides within an interstice formed byproximal glass microspheroids according toD≦2√{square root over (2 r ²)}−2 r where r is the average radius of fourcontiguous microspheroids; such that cure of said resin yields anarticle having a linear shrinkage of less than ±0,06%, a density of lessthan 1.65, and the smoothness of a class-A surface finish.
 14. Theformulation of claim 13, wherein said glass microspheroids are presentfrom 4 to 6 total weight percent of the formulation.
 15. The formulationof claim 13, further comprising a surface activating agent bonded to thesurface of said glass microspheroids.
 16. The formulation of claim 15,wherein said surface activating agent is a silane.
 17. The formulationof claim 15 wherein said surface activating agent further comprises across-linkable moiety reactive during the cure of said resin.
 18. Theformulation of claim 15 wherein said surface activating agent is aheteroatom functionally terminated thermoplastic coating.
 19. A processfor producing an article from a molding compound formulation of claim 1and retaining surface quality comprising: adding a surface activatingagent coated glass microspheroid having a mean diameter of from 16 to 35microns to a molding composition formulation containing a thermosetcross-linkable polymeric resin; and allowing sufficient time for saidresin to cross link.
 20. The process of claim 19 wherein said surfaceactivating agent is reactive and cross links to said thermosetcross-linkable resin.